Optical disk drives provide for the storage of great quantities of data on a disk. The data is accessed by focusing a laser beam onto the data layer of the disk and then detecting the reflected light beam.
Magneto-optical (M-O) systems write data by directing a laser to a spot on the data layer to heat it above its Curie temperature while the magnetic domain of the spot is oriented in either an up or a down direction by an external magnetic field. The data is read by directing a low power laser to the data layer. The differences in magnetization direction of the spots cause the plane of polarization of the reflected light beam to be rotated either clockwise or counterclockwise. This change in orientation of polarization of the reflected light is then detected. Magnetic super resolution (MSR) M-O media operates in the same manner as conventional M-O media but uses at least two magnetically coupled magnetic layers and requires a much higher laser power to read the data. Direct overwrite (DOW) M-O media uses at least two magnetically coupled magnetic layers and allows erasure of data and writing of new data to occur in the same disk rotation.
Phase change systems write data by directing the laser to a spot on the data layer to cause a structural change of the data layer, typically from a crystalline phase to an amorphous phase. The data is detected as changes in reflectivity as the laser beam passes over the different phases. Alloying systems write data by the heating of two chemically distinct materials, such as Bi.sub.2 Te.sub.3 and Sb.sub.2 Se.sub.3, to form an amorphous alloy in the data layer. In alloying systems the data is detected as changes in reflectivity. Phase change media and alloying media are used as write-once read many (WORM) media.
In all of these types of systems the writing of data thus occurs due to laser heating of the material in the data layer.
Pulse width modulation (PWM) is one way to write data as marks on optical disks. In PWM, a mark can be either an individual spot (also called a submark) or a series of overlapping or contiguous submarks. PWM records information as the distance between the transitions or edges of the marks. A transition is either the beginning (leading) or end (trailing) edge of a mark. PWM recording is difficult to implement because the mark edges must be precisely positioned and written with sharp boundaries to ensure accurate recording. The thermal buildup that occurs within the data layer in the gaps between the trailing edges and leading edges of adjacent marks during the laser writing process can cause great distortions in the precise placement of the leading edges. Thermal buildup occurs when there is insufficient time between the writing of successive submarks in a mark to allow the data layer to cool prior to the writing of the next mark. Both an increase in disk drive data rate and an increase in linear data density on the disk are causes for the insufficient cooling time. In addition to this problem of thermal preheating caused by thermal buildup, the gap may be so long that there is excessive cooling of the data layer so that the data layer does not reach the required temperature at the precise time to write the submark forming the leading edge of the next mark. In PWM writing the gap lengths also vary so the effect of thermal preheating and cooling on placement of the subsequent mark leading edges depends on the type of mark previously written as well as the length of the gap. Under these conditions of thermal preheating or cooling of the data layer caused by the prior write history, errors occur in the placement of the leading edges of marks. The problem of precise placement of mark edges becomes more significant as the linear density of the submarks increases and the spacing between the submarks decreases because the peak temperature of the thermal interaction in the data layer increases. In addition, each type of optical media has its own thermal characteristics so the problems of thermal preheating and cooling will vary depending on the type of media being used.
What is needed is a PWM optical disk drive that reliably writes marks on the disk so that the mark edges are precisely aligned without the adverse effect of thermal preheating or cooling, regardless of the prior pattern of written marks and gaps.